I made myself unpopular with the demonstrators as a 1st year undergraduate because, faced with building an op amp using 2N3819 in the input stage, I asked if I could use 12AX7s instead (200VDC was available on the lab benches.) I also had a good 12AX7 and base handy.
Germanium transistors were basically made by diffuse and hope, and were individual dies. Once planar silicon arrived you could be reasonably sure of getting matched pairs so long as your lot all came off the same part of the same wafer. AD used to make extremely close single die matched pair for custom circuits and as far as I know they may still. The difficulty with lots of feedback is that it is fine if you want to, say, build a thermometer, but for an analog computer with any frequency response the current through the matched pair will vary, causing differential heating and a rapid change in Vbe, and bang goes your offset. I remember giving up on trying to help a research student in the early 70s - the issue was that the output went to a plotter several rooms away, and the drift was such that by the time the plotter was zeroed the circuit was already drifting. There was also no telephone link between the rooms. I could have replicated that experiment nowadays with a few op amps and a single PIC microprocessor connected via USB to a laptop, instead of it being spread all over a bench.
